Add support for -ctrl-grab to use the right-ctrl button to grab/release
the mouse in SDL.

The multi-button ctrl-alt and ctrl-alt-shift grab buttons present an
accessibility problem to users who cannot press more than one button
at a time.

https://bugs.edge.launchpad.net/ubuntu/+source/qemu-kvm/+bug/237635Signed-off-by: NDustin Kirkland <kirkland@canonical.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>